The present invention relates to a method of and an arrangement for producing of hollow injected shaped bodies of synthetic plastic material.
Hollow injection processes for producing synthetic plastic shaped bodies to be used in the automobile industry for example central consoles, coating parts, arm rests, etc. are known in various forms. In accordance with one known process disclosed for example in the German document DE-OS 2,501,314, the gas, after filling the mold with a synthetic plastic melt, is introduced by an injection nozzle directly into the synthetic plastic mold. After a sufficient cooling time, the injection nozzle is separated from the mold and thereby the hollow of the shaped body is open to atmosphere. After discharging the gas from the hollow of the shaped body and a pressure equalization with the atmosphere, the mold is open and the finished shaped body is removed from the mold.
In the known processes, the forming of the shaped body is performed by a stationary pressure cushion formed by the gas during the whole process time. Since the gas cannot contribute to a cooling of the shaped hollow body due to a static pressure cushion, the cooling time per injection cycle in this process is relatively long and therefore the output of such a device is relatively low. This device has an especially grave disadvantage when, due to the construction of the shaped body parts, the inner cross-section is formed as a blind hole which during filling is forcedly supplied with the synthetic plastic melt and therefore the stationary gas pressure cushion cannot contribute to the desired hollow formation of the shaped body in the region of the blind hole cross-section. The thusly produced uncontrollable material accumulation in this cross-sectional region poses the danger of forming sink marks due to a non-uniform wall thickness of the shaped body. A further disadvantage is connected with the non-uniform cooling of the outer wall of the shaped body by the adjoining molding tool relative to the wall of the hollow of the shaped body, due to the stationary gas pressure cushion. This leads to distortions and material shrinkage zones in the shaped body and as a result to a lower quality of the finished product, which then requires subsequent expensive surface treatments.